A fuel cell, for example a polymer electrolyte fuel cell, is an apparatus that allows a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air to electrochemically react with each other at a gas diffusion layer that has a catalyst layer such as platinum, such that electric power and heat are produced at the same time.
FIG. 8 is a schematic diagram showing the basic structure of a conventional polymer electrolyte fuel cell. A single cell (also referred to as a cell) 100 of the polymer electrolyte fuel cell includes a membrane electrode assembly 110 (hereinafter referred to as the MEA: Membrane-Electrode-Assembly) and paired plate-like electrically conductive separators 120 disposed on the opposite faces of the MEA 110, respectively.
The MEA 110 includes a polymer electrolyte membrane (a resin ion exchange membrane) 111 that selectively transports hydrogen ions, and paired electrode layers 112 formed at the opposite faces of the polymer electrolyte membrane 111. The paired electrode layers 112 are formed at the opposite faces of the polymer electrolyte membrane 111, and each include a catalyst layer 113 which is mainly comprised of carbon powder bearing a platinum metal catalyst, and a gas diffusion layer 114 that is formed on the catalyst layer 113 and that has combined features of current-collecting effect, gas permeability, and water repellency. The gas diffusion layer 114 is structured with a porous base material 115 made of carbon fibers, and a coating layer (a water-repellent carbon layer) 116 made of carbon and a water-repellent member.
The paired separators 120 are provided with, at their main surfaces abutting on the gas diffusion layers 114, respectively, fuel gas flow passages 121 for allowing the fuel gas to flow through, and oxidant gas flow passages 122 for allowing the oxidant gas to flow through. Further, the paired separators 120 are provided with coolant flow channels 123 through which coolant or the like flows through. Supply of the fuel gas and the oxidant gas to the paired electrode layers 112 through the gas flow passages 121 and 122, respectively, causes an electrochemical reaction, to produce electric power and heat.
As shown in FIG. 8, the cell 100 structured as described above is generally used by being stacked by one piece or more, so that the cells 100 adjacent to each other are electrically connected in series. It is noted that, here, the cells 100 stacked together are fastened under pressure at a prescribed fastening pressure by fastening members 130 such as bolts, so as to prevent leakage of the fuel gas and the oxidant gas, which are the reactant gas, and to reduce the contact resistance. Accordingly, each of the MEAs 110 and each of the separators 120 are brought into plane-to-plane contact at a prescribed pressure. Here, the separators 120 have a current collecting ability for electrically connecting adjacent ones of the MEAs 110 and 110 in series. Further, in order to prevent the gases required for the electrochemical reaction from leaking externally, sealing members (gaskets) 117 are disposed between the paired separators 120 and 120 so as to cover the side surfaces of the catalyst layer 113 and the gas diffusion layer 114.
In recent years, in the field of the fuel cell, there is a demand for a further reduction in costs. Accordingly, from the viewpoint of a reduction in the unit price of the constituents and in the number of components, various techniques for achieving a reduction in costs have been proposed. One of such proposals is a technique of providing the gas flow passages at the gas diffusion layer, instead of at the separator.
In the conventional fuel cell shown in FIG. 8, the gas flow passages are provided at each of the separators. A method of implementing such a structure is to use, e.g., carbon and resin, as the material of each separator, and subjecting them to injection molding using a mold having concavity and convexity corresponding to the shape of the gas flow passages. However, in this case, there is an issue of the high production cost. Further, another method of implementing such a structure is to use metal as the material of each separator, and to roll the metal using a mold having concavity and convexity corresponding to the shape of the gas flow passages. However, in this case, though a reduction in cost can be realized as compared to the injection molding, there is an issue that the separator is prone to corrode, which in turn impairs the power generation performance as the fuel cell.
On the other hand, the gas diffusion layer is structured with a porous member, such that it possesses gas diffusibility. Accordingly, it is easier to form the gas flow passages at the gas diffusion layer than forming them at the separator, and is advantageous in reducing the cost and in achieving higher power generation performance. For example, Patent Documents 1 to 3 each disclose a gas diffusion layer having such a structure.
Patent Document 1 discloses the following technique: using a molding jig provided with a plurality of flow channel molds each elongated in a rectangular parallelepiped form, molding a porous member whose base material is carbon fibers by a paper-making method; and thereafter, by removing the molding jig, forming gas flow passages inside the gas diffusion layer.
Patent Document 2 discloses the following technique: patterning partition walls made of resin or metal which are to form the gas flow passages on a separator; and thereafter, shaping a porous member whose base material is carbon fibers so as to cover the partition walls, to form the gas flow passages at a gas diffusion layer.
Patent Document 3 discloses the following technique: disposing a flow channel structuring member made of carbon paper provided with a gas flow passages structure by punching or the like between a porous member whose base material is carbon fibers, and a flat plate-like separator, to form gas flow passages at a gas diffusion layer.
Further, as disclosed in the aforementioned Patent Documents 1 to 3, the gas diffusion layer is generally structured with a porous member whose base material is carbon fibers. However, the porous member whose base material is carbon fibers involves complicated production steps and requires a considerable production cost, and hence, it is expensive. Accordingly, a technique has been proposed to achieve a reduction in costs of the fuel cell by structuring a gas diffusion layer without using the porous member whose base material is carbon fibers. As the gas diffusion layer of such a structure, for example, Patent Document 4 discloses one.
Patent Document 4 discloses a technique of structuring a gas diffusion layer by mixing graphite, carbon black, uncalcined PTFE (polytetrafluoroethylene), and calcined PTFE, without using carbon fibers as the base material.